1. Field of the Invention
The present invention concerns a weighing method for the prepla type injection molding machine comprising both plasticating apparatus and an injection apparatus.
2. Detailed Description of the Prior Art
In the prepla type injection molding machine, plastic resin by screw rotation is supplied to the injection cylinder forward portion from the plastication cylinder through a resin path. Plastic resin is weighed by accumulation in the forward portion of the injection cylinder by retracting the plunger at the weighing start position by resin pressure. The plunger is advanced by blocking the resin path after the weighing. Plastic resin is injected and charged into the die from an injection cylinder end nozzle.
The plunger after this injection and filling is in the most advanced position thereof, but the plunger retracts slightly to the place where a pressure is mitigated by resin counterforce generated by decompression after the completion of injection pressure-keeping, and almost at the same time, the resin path is opened to start supplying plastic resin.
The plunger retraction weighing only by resin pressure makes the density of plastic resin weighed there unstable; therefore, a fixed backpressure is applied to the plunger, and the plastic resin supply is stopped, when the plunger attains a predetermined weighing stop position under this backpressure control.
For the aforementioned screw, there are both a type wherein resin is plasticized by rotating at a fixed position and, at the same time, plastic resin is supplied to the injection cylinder by discharge pressure, and a inline screw method where plastic resin is accumulated in the screw forward portion by retracting under resin pressure all the way rotating, and supplied by forward movement. In either of these types, plastic resin supply starts by opening the resin path.
In such method of the prior art, the resin weighing (weighing time, weighing density, resin temperature, or other) may become irregular, often making it difficult to obtain a good product continuously. It is supposed that, according to resins, a minute change of sliding resistance or others of the whole apparatus retracted by the plunger sliding resistance or resin supplied from the screw provokes backpressure variation, and changes the plastic resin feed state by screw rotation or forward movement.
The present invention is devised to solve the problem of the weighing discrepancy of the aforementioned prior art and has as an object to provide a weighing method for the prepla type injection molding machine that can cancel the influence of the plunger sliding resistance and stabilize the weighing, by controlling the plunger retraction movement for the weighing by dividing the control into speed control and pressure control.
The present invention according to the aforementioned object, is a weighing method for a prepla type injection molding machine, comprising both a plasticating cylinder with an internal screw and an injection cylinder with an internal plunger, for supplying a forward portion of the injection cylinder with plastic resin by screw revolution or advancement, and injecting plastic resin by advance of the plunger after blocking the resin passage, comprising the steps of:
setting sequentially a plastic resin supply start position and a forced retraction release position along the way of the plunger retraction stroke by reference to a weighing start position;
weighing plastic resin by moving the aforementioned plunger from a weighing start position after injection and pressure-keeping to a weighing stop retraction position and starting to supply the forward portion of the injection cylinder with plastic resin when the plunger attains the supply start position from the weighing start position, by forced retraction of the plunger up to the forced retraction release position thereof by speed control; and
stabilizing the weighing by changing the speed control to the pressure control once the plunger attains the forced retraction release position.